Semiconductor device

ABSTRACT

A semiconductor device includes: a semiconductor layer that includes principal surfaces; a metal layer that includes principal surfaces, is disposed with the principal surface in contact with the principal surface, is thicker than the semiconductor layer, and comprises a first metal material; a metal layer that includes principal surfaces, is disposed with the principal surface in contact with the principal surface, and comprises a metal material having a Young&#39;s modulus greater than that of the first metal material; and transistors. The transistor includes a source electrode and a gate electrode on a side facing the principal surface. The transistor includes a source electrode and a gate electrode on a side facing the principal surface.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is a Continuation application of U.S. patentapplication Ser. No. 16/488,199, filed on Aug. 22, 2019, which is theU.S. National Phase under 35 U.S.C. § 371 of International PatentApplication No. PCT/JP2019/001315, filed on Jan. 17, 2019 which in turnclaims the benefit of U.S. Provisional Application No. 62/687,051, filedon Jun. 19, 2018, the entire disclosures of which Applications areincorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to semiconductor devices, and inparticular to chip-size-package-type semiconductor devices that arefacedown mountable.

BACKGROUND ART

Conventionally, a semiconductor device has been proposed that includes:a semiconductor layer having a first principal surface and a secondprincipal surface; two vertical field-effect transistors providedextending from the first principal surface to the second principalsurface, and a metal layer formed on the second principal surface. Thisconfiguration allows not only a horizontal path in the semiconductorsubstrate, but a horizontal path in the metal layer, where conductionresistance is low, to be used as a path along which current flows fromthe first transistor to the second transistor, whereby the on-resistanceof the semiconductor device can be lowered.

Patent Literature (PTL) 1 proposes a flip-chip semiconductor devicewhich has, in addition to the above configuration, a conductive layerformed on a side of the metal layer that is the side opposite to thesemiconductor substrate. The conductive layer can inhibit burrs of themetal layer from forming in the chip singulation step.

PTL 2 proposes a flip-chip semiconductor device which has, in additionto the above configuration, an insulating coating formed on a side ofthe metal layer that is the side opposite to the semiconductorsubstrate. The insulating coating can prevent scratches, breaks, andother damages while keeping the semiconductor device thin.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2016-86006

PTL 2: Japanese Unexamined Patent Application Publication No.2012-182238

SUMMARY OF THE INVENTION Technical Problems

However, in the semiconductor devices disclosed in PTL 1 and PTL 2, thecoefficient of thermal expansion of the metal layer is greater than thecoefficient of thermal expansion of the semiconductor substrate, whichcauses the semiconductor device to warp from changes in temperature.

In PTL 1, the conductive layer is formed on a side of the metal layerthat is the side opposite to the semiconductor substrate, but since theconductive layer primarily contains the same metal contained in themetal layer, from a manufacturing viewpoint, it is not easy to form aconductive layer thick enough to reduce the warpage of the semiconductordevice from changes in temperature.

In PTL 2, an insulating coating for reducing the thickness of thesemiconductor device and preventing damage of the semiconductor deviceis formed on a side of the metal layer that is the side opposite to thesemiconductor substrate, but no stress great enough to reduce thewarpage of the semiconductor device is induced in the insulating coatingwhen the metal layer is formed at a thickness required to ensure lowon-resistance.

That is, the semiconductor devices disclosed in PTL 1 and PTL 2 cannotlower the on-resistance and suppress warpage of the semiconductor deviceat the same time.

In view of this, the present disclosure has an object to provide achip-size-package-type semiconductor device that allows both reductionin on-resistance and suppression of warpage.

Solutions to Problems

In order to solve the above problems, a semiconductor device accordingto one aspect of the present disclosure is a chip-size-package-typesemiconductor device that is facedown mountable, the semiconductordevice including: a semiconductor layer that includes a first principalsurface and a second principal surface that face in opposite directions,and comprises silicon, gallium nitride, or silicon carbide; a firstmetal layer that includes a third principal surface and a fourthprincipal surface that face in opposite directions, is disposed with thethird principal surface in contact with the second principal surface, isthicker than the semiconductor layer, and comprises a first metalmaterial; a second metal layer that includes a fifth principal surfaceand a sixth principal surface that face in opposite directions, isthicker than the semiconductor layer, and comprises a second metalmaterial having a Young's modulus greater than a Young's modulus of thefirst metal material; a first vertical field effect transistor disposedin a first region of the semiconductor layer; and a second verticalfield effect transistor disposed in a second region adjacent to thefirst region in a direction along the first principal surface in thesemiconductor layer. The first vertical field effect transistor includesa first source electrode and a first gate electrode on a side facing thefirst principal surface of the semiconductor layer. The second verticalfield effect transistor includes a second source electrode and a secondgate electrode on the side facing the first principal surface of thesemiconductor layer. The first metal layer functions as a common drainelectrode for the first vertical field effect transistor and the secondvertical field effect transistor. A bidirectional path from the firstsource electrode to the second source electrode via the common drainelectrode is a primary current path.

The configuration described above, in which the first metal layer havinga thickness for ensuring low on-resistance is in contact with the secondmetal layer having a Young's modulus greater than that of the firstmetal layer and thicker than the semiconductor layer, can suppresswarpage of the semiconductor device that occurs due to the contactbetween the semiconductor layer and the first metal layer. Achip-size-package-type semiconductor device that allows both reductionin on-resistance and suppression of warpage can therefore be provided.

Advantageous Effects of Invention

With the semiconductor device according to the present disclosure, it ispossible to provide a chip-size-package-type semiconductor device thatcan both reduce on-resistance and suppress warpage of the semiconductordevice.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing an example of the configurationof a semiconductor device according to an embodiment.

FIG. 2 includes a top view illustrating one example of an electrodeconfiguration of a semiconductor device according to an embodiment and across-sectional schematic view illustrating the flow of bidirectionalcurrent in the semiconductor device.

FIG. 3 is a circuit diagram illustrating an example of an application ofa semiconductor device according to an embodiment in a charge/dischargecircuit.

FIG. 4A shows a graph illustrating the amount of warpage and themagnitude of on-resistance of a semiconductor device having a layeredconfiguration formed of an Si layer/Ag layer versus the thickness of anAg layer/thickness of an Si layer.

FIG. 4B shows a graph of results of a prototype experiment for checkingthe amount of warpage of the semiconductor device having a layeredconfiguration formed of an Si layer/Ag layer/Ni layer versus thethickness of the Ni layer.

FIG. 5A shows a graph of results of a prototype experiment for checkingthe on-resistance of the semiconductor device having the layeredconfiguration formed of the Si layer/Ag layer/Ni layer or the Silayer/Ag layer versus the thickness of the Si layer.

FIG. 5B shows a graph of results of a prototype experiment for checkingthe on-resistance of the semiconductor device having the layeredconfiguration formed of the Si layer/Ag layer/Ni layer versus thethickness of the Ni layer.

(a) in FIG. 6 is an electron micrograph of a principal surface of the Nilayer in the semiconductor device according to an embodiment, and (b) inFIG. 6 is an electron micrograph of a cross section of the Ni layer inthe semiconductor device according to the embodiment.

FIG. 7 shows electron micrographs each illustrating a cross section ofthe Ni layer/Ag layer in the semiconductor device according to anembodiment.

FIG. 8 shows a graph illustrating comparison between actually measuredvalues and estimated values of the amount of warpage of thesemiconductor device having the layered configuration formed of the Silayer/Ag layer/Ni layer versus the thickness of the Ni layer.

FIG. 9 is a schematic cross-sectional view of semiconductor devices eachincluding an Ni layer formed of a plurality of layers containingdifferent crystal grain sizes.

FIG. 10 shows the temperature dependence of the amount of warpage of asemiconductor device including an Ni layer formed of two layers formedby using different plating methods.

FIG. 11 is a schematic cross-sectional view showing the relationship ofthe cycle of irregularities of a principal surface of the Ni layer andthe width of a mark pattern with the visibility of the mark.

FIG. 12 is a schematic cross-sectional view showing the relationship ofthe mean roughness depth of the principal surface of the Ni layer andthe depth of the mark with the visibility of the mark.

FIG. 13A is a cross-sectional view of a semiconductor device accordingto an embodiment.

FIG. 13B is a cross-sectional view of a semiconductor device accordingto an embodiment.

FIG. 13C is a cross-sectional view of a semiconductor device accordingto an embodiment.

FIG. 13D is a cross-sectional view of a semiconductor device accordingto an embodiment.

FIG. 13E is a cross-sectional view of a semiconductor device accordingto an embodiment.

FIG. 13F is a cross-sectional view of a semiconductor device accordingto an embodiment.

FIG. 14 is a cross-sectional view of a semiconductor device according toan embodiment.

FIG. 15 describes a setback distance of the Si layer in a semiconductordevice according to an embodiment.

FIG. 16 is an electron micrograph of the side surface of the Si layer ina semiconductor device according to an embodiment.

FIG. 17 describes the relationship between the shape of the side surfaceof the Si layer and the method for manufacturing the Si layer in asemiconductor device according to an embodiment.

FIG. 18 is an electron micrograph of the side surface of the Si layer/Aglayer in a semiconductor device according to an embodiment.

FIG. 19 is a cross-sectional view of a semiconductor device according toan embodiment.

FIG. 20 is a schematic cross-sectional view of a semiconductor deviceaccording to an embodiment.

FIG. 21 is a schematic cross-sectional view of a semiconductor deviceaccording to an embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following embodiment is a specific example of the presentdisclosure. The numerical values, shapes, materials, elements,arrangement and connection configuration of the elements, etc.,described in the following embodiment are merely examples, and are notintended to limit the present disclosure. Among elements in thefollowing embodiment, those not described in any one of the independentclaims indicating the broadest concept of the present disclosure aredescribed as optional elements.

Embodiment [1. Configuration of Semiconductor Device]

Hereinafter, the configuration of semiconductor device 1 according tothe present embodiment will be described. Semiconductor device 1according to the present disclosure is a facedown mountable,chip-size-package (CSP) type multi-transistor chip including twovertical metal oxide semiconductor (MOS) transistors formed on asemiconductor substrate. The two vertical MOS transistors are each apower transistor and what is called a trench MOS field effect transistor(FET). Semiconductor device 1 according to the present embodiment is,however, not used in a device that belongs to optoelectronics products,such as a solid-state imaging device.

FIG. 1 is a cross-sectional view showing an example of the configurationof semiconductor device 1 according to an embodiment. FIG. 2 includes atop view illustrating one example of an electrode configuration ofsemiconductor device 1 according to an embodiment and a cross-sectionalschematic view illustrating the flow of bidirectional current insemiconductor device 1 according to an embodiment. The cross-sectionalview of FIG. 1 shows the plane taken along line I-I in (a) in FIG. 2.

As illustrated in FIG. 1, semiconductor device 1 includes semiconductorlayer 40, metal layers 30 and 31, first vertical MOS transistor 10(hereinafter referred to as transistor 10), and second vertical MOStransistor 20 (hereinafter referred to as transistor 20).

Semiconductor layer 40 includes principal surface 40 a (first principalsurface) and principal surface 40 b (second principal surface) that facein opposite directions, and comprises silicon. Semiconductor layer 40has a configuration in which semiconductor substrate 32 andlow-concentration impurity layer 33 are layered on each other.Semiconductor substrate 32 is disposed on the side facing principalsurface 40 b of semiconductor layer 40, and low-concentration impuritylayer 33 is disposed on the side facing principal surface 40 a ofsemiconductor layer 40.

Metal layer 31 is a first metal layer that includes principal surface 31a (third principal surface) and principal surface 31 b (fourth principalsurface) that face in opposite directions, is disposed with principalsurface 31 a in contact with principal surface 40 b, is thicker thansemiconductor layer 40, and comprises a first metal material. Examplesof the first metal material may include silver (Ag), copper (Cu), andgold (Au).

Metal layer 30 is a second metal layer that includes principal surface30 a (fifth principal surface) and principal surface 30 b (sixthprincipal surface) that face in opposite directions, is disposed withprincipal surface 30 a in contact with principal surface 31 b, isthicker than semiconductor layer 40, and comprises a second metalmaterial having a Young's modulus greater than that of the first metalmaterial. Examples of the second metal material may include nickel (Ni),platinum (Pt), iridium (Ir), rhodium (Rh), and chromium (Cr).

Moreover, as illustrated in FIG. 1 as well as (a) and (b) in FIG. 2, ina plan view of semiconductor layer 40, transistor 10 formed in firstregion A1 includes, on the principal surface 40 a side of semiconductorlayer 40, four source electrodes 11 a, 11 b, 11 c, and 11 d (eachcorresponding to one of source electrodes 11), and one gate electrode 19(first gate electrode). Moreover, transistor 20 formed in second regionA2 adjacent to first region A1 in a direction along principal surface 40a includes four source electrodes 21 a, 21 b, 21 c, and 21 d (eachcorresponding to one of source electrodes 21), and one gate electrode 29(second gate electrode). The number of source electrodes and gateelectrodes included in one transistor 10 and one transistor 20 and thearrangement of the source and gate electrodes are not limited to thoseshown in FIG. 2.

Metal layer 31 functions as a common drain electrode for transistors 10and 20, as shown in (b) and (c) in FIG. 2, and a bidirectional path fromsource electrodes 11 (first source electrode) to source electrodes 21(second source electrode) via metal layer 31 is a primary current path.

The configuration described above, in which metal layer 31 having athickness for ensuring low on-resistance is in contact with metal layer30 having a Young's modulus greater than that of metal layer 31 andthicker than semiconductor layer 40, can suppress warpage ofsemiconductor device 1 that occurs due to the contact betweensemiconductor layer 40 and metal layer 31. Chip-size-package-typesemiconductor device 1 that allows both reduction in on-resistance andsuppression of warpage can therefore be provided.

The configuration and effects of semiconductor device 1 will bedescribed below in detail.

Semiconductor substrate 32 contains an impurity of a first conductivitytype and comprises silicon. Semiconductor substrate 32 is, for example,an n-type silicon substrate.

Low-concentration impurity layer 33 is formed to be in contact with theupper surface (principal surface facing positive side of axis z inFIG. 1) of semiconductor substrate 32 and contains an impurity of thefirst conductivity type in lower a concentration than the concentrationof an impurity of the first conductivity type in semiconductor substrate32. Low-concentration impurity layer 33 may be formed on semiconductorsubstrate 32, for example, in an epitaxial growth process.

Body region 18, which contains an impurity of a second conductivity typedifferent from the first conductivity type, is formed in first region A1of low-concentration impurity layer 33. Source region 14 containing animpurity of the first conductivity type, gate conductor 15, and gateinsulating film 16 are formed in body region 18. Source electrode 11 isformed of portion 12 and portion 13, and portion 12 is connected tosource region 14 and body region 18 via portion 13. Gate conductor 15 isconnected to gate electrode 19.

Portion 12 of source electrode 11 is a layer showing satisfactorybondability to an electrically conductive bonding material, such assolder, in an implementation process and may comprise a metal materialcontaining at least one of nickel, titanium, tungsten, and palladium, asa non-limiting example. The surface of portion 12 may be plated, forexample, with gold.

Portion 13 of source electrode 11 is a layer that connects portion 12and semiconductor layer 40 to each other and may comprise a metalmaterial containing at least one of aluminum, copper, gold, and silver,as a non-limiting example.

Body region 28, which contains an impurity of the second conductivitytype different from the first conductivity type, is formed in secondregion A2 of low-concentration impurity layer 33. Source region 24containing an impurity of the first conductivity type, gate conductor25, and gate insulating film 26 are formed in body region 28. Sourceelectrode 21 is formed of portion 22 and portion 23, and portion 22 isconnected to source region 24 and body region 28 via portion 23. Gateconductor 25 is connected to gate electrode 29.

Portion 22 of source electrode 21 is a layer showing satisfactorybondability to an electrically conductive bonding material, such assolder, in an implementation process and may comprise a metal materialcontaining at least one of nickel, titanium, tungsten, and palladium, asa non-limiting example. The surface of portion 22 may be plated, forexample, with gold.

Portion 23 of source electrode 21 is a layer that connects portion 22and semiconductor layer 40 to each other and may comprise a metalmaterial containing at least one of aluminum, copper, gold, and silver,as a non-limiting example.

Body regions 18 and 28 are covered with interlayer insulating film 34having openings, and portions 13 and 23, which are part of the sourceelectrodes and connected to source regions 14 and 24, respectively, viathe openings of interlayer insulating film 34, are provided. Interlayerinsulating film 34 and portions 13 and 23 of the source electrodes arecovered with passivation layer 35 having openings, and portions 12 and22, which are connected to portions 13 and 23 of the source electrodes,respectively, via the openings of passivation layer 35, are provided.

[2. Operation of Semiconductor Device]

In semiconductor device 1 shown in FIG. 1, for example, assuming thatthe first conductivity type is the n type and the second conductivitytype is the p type, source regions 14 and 24, semiconductor substrate32, and low-concentration impurity layer 33 may comprise an n-typesemiconductor, and body regions 18 and 28 may comprise a p-typesemiconductor.

Moreover, for example, assuming that the first conductivity type is thep type and the second conductivity type is the n type, source regions 14and 24, semiconductor substrate 32, and low-concentration impurity layer33 may comprise a p-type semiconductor, and body regions 18 and 28 maycomprise an n-type semiconductor.

In the following description, the electric conduction operation ofsemiconductor device 1 will be described with reference to what iscalled an N-channel transistor with the first conductivity type beingthe n type and the second conductivity type being the p type.

In semiconductor device 1 shown in FIG. 1, when high voltage is appliedto source electrodes 11 and low voltage is applied to source electrodes21, and voltage greater than or equal to a threshold is applied to gateelectrode 29 (gate conductor 25) with respect to the voltage at sourceelectrode 21, a conduction channel is formed in the vicinity of gateinsulating film 26 in body region 28. As a result, on-current flowsthrough the following path: source electrode 11-body region18-low-concentration impurity layer 33-semiconductor substrate 32-metallayer 31-semiconductor substrate 32-low-concentration impurity layer33-the conduction channel formed in body region 28-source region24-source electrode 21, so that semiconductor device 1 is electricallyconductive. PN junction is present in the conduction path at the planewhere body region 18 and low-concentration impurity layer 33 are incontact with each other, and the PN junction functions as a body diode.Since the on-current flows through metal layer 31, increasing thethickness of metal layer 31 increases the cross-sectional area of theon-current path, whereby the on-resistance of semiconductor device 1 canbe lowered. The electrical conduction state is the charge state in FIG.3, which will be described later.

[3. Configuration that Achieves Both Warpage Reduction and LowOn-Resistance of Semiconductor Device]

FIG. 3 is a circuit diagram illustrating an example of an application ofsemiconductor device 1 in a charge/discharge circuit of a smartphone ortablet. Semiconductor device 1 controls discharging operations frombattery 3 to load 4 and charging operations from load 4 to battery 3depending on the control signal applied by control IC 2. Whensemiconductor device 1 is implemented as a charge/discharge circuit in asmartphone or tablet in this way, the on-resistance is required to belower than or equal to a value in a range of from 2.2 mΩ to 2.4 mΩ as a20 V withstand voltage specification, due to a short charge period,rapid charging, and other restrictions.

In a case where semiconductor device 1 is mounted on a mountingsubstrate, source electrodes 11, gate electrode 19, source electrodes21, and gate electrode 29 are bonded to electrodes provided on themounting substrate via an electrically conductive bonding material, suchas solder, in facedown mounting. In this case, as the warpage ofsemiconductor device 1 increases, the electrical connection of sourceelectrodes 11, gate electrode 19, source electrodes 21, and gateelectrode 29 to the electrodes provided on the mounting substratebecomes less stable. That is, to further stabilize the bonding betweensemiconductor device 1 and the electrodes on the mounting substrate, thewarpage of semiconductor device 1 needs to be reduced.

FIG. 4A shows a graph of results of a prototype experiment for checkingthe amount of warpage and the magnitude of on-resistance of asemiconductor device having a layered configuration formed ofsemiconductor layer 40 (hereinafter referred to as Si layer in somecases)/metal layer 31 (hereinafter referred to as Ag layer in somecases) versus the thickness of the Ag layer/thickness of the Si layer(quotient of division of thickness of Ag layer by thickness of Silayer). More specifically, FIG. 4A shows the amount of warpage at 250°C. and the magnitude of on-resistance of the semiconductor device havinga longer-side length of 3.40 mm (L1 in FIG. 2) and a shorter-side lengthof 1.96 mm (L2 in FIG. 2). FIG. 4A indicates that the thickness of theAg layer/thickness of the Si layer that achieves on-resistance lowerthan or equal to 2.4 mΩ needs to be greater than 1.0. On the other hand,over the range where the thickness of the Ag layer/thickness of the Silayer is greater than 1.0, the amount of warpage at 250° C. is notsmaller than or equal to 60 μm, which is the industry standard.

In contrast, metal layer 30 (hereinafter referred to as Ni layer in somecases) is provided to suppress the warpage of semiconductor device 1with low on-resistance of semiconductor device 1 ensured. This is astructure in which semiconductor layer 40 and metal layer 30 sandwichmetal layer 31, and from the viewpoint of balance of the stress at theopposite surfaces of metal layer 31, it is desirable for suppression ofthe amount of warpage that the material of metal layer 30 has physicalproperties comparable to those of the material of semiconductor layer 40and the thickness of metal layer 30 is comparable to that ofsemiconductor layer 40. However, such a metal material does not exist,and it is therefore at least necessary that the material of metal layer30 has physical properties closer to those of semiconductor layer 40than those of metal layer 31, and that metal layer 30 is thicker thansemiconductor layer 40.

Table 1 shows typical film thicknesses and physical properties of the Silayer, the Ag layer, and the Ni layer, which are examples ofsemiconductor layer 40, metal layer 31, and metal layer 30.

TABLE 1 Coefficient Film Young's of thermal thickness modulus expansionManufacturing (μm) (GPa) (ppm) method Si layer 20 (t1) 185 (E1) 3-5 (α1)Ag layer 50 (t2) 83 (E2) 18.9 (α2) Electroplating Ni layer 30 (t3) 200(E3) 12.8 (α3) Electroplating t1 < t2 E3 > E2 α3 < α2 t1 < t3

A Young's modulus of the second metal material included in the Ni layeris greater than a Young's modulus of the first metal material includedin the Ag layer, as shown in Table 1. The Ni layer is thicker than theSi layer, and the Ag layer is thicker than the Si layer. Further, thecoefficient of thermal expansion of the second metal material includedin the Ni layer is less than the coefficient of thermal expansion of thefirst metal material included in the Ag layer. The warpage ofsemiconductor device 1 can be further suppressed because the coefficientof thermal expansion of the Ni layer is less than the coefficient ofthermal expansion of the Ag layer.

FIG. 4B shows a graph of results of a prototype experiment for checkingthe amount of warpage of semiconductor device 1 having the layeredconfiguration formed of the Si layer/Ag layer/Ni layer versus thethickness of the Ni layer. More specifically, FIG. 4B shows a graph ofcomputer-simulated results of expected amounts of warpage at 250° C.versus the thickness of the Ni layer in a case where the Si layer has athickness of 20 μm and the Ag layer has a thickness of 50 μm.

In a state in which no Ni layer is present (thickness of Ni layer=0 μm),the amount of warpage is about 67 μm, but the amount of warpagedecreases as the thickness of the Ni layer increases, as shown in FIG.4B. To fully suppress the warpage-related mounting problem, the amountof warpage needs to be reduced to about 30 μm. To this end, the Ni layeris desirably thicker than the Si layer.

The results shown in FIGS. 4A and 4B indicate that, to achieve both thereduction in the amount of warpage and low on-resistance, the Ag layerneeds to be thicker than the Si layer and the Ni layer needs to bethicker than the Si layer in semiconductor device 1 having the layeredconfiguration formed of the Si layer/Ag layer/Ni layer.

The layered configuration that can achieve both the reduction in theamount of warpage and low on-resistance will next be described.

FIG. 5A shows a graph of results of a prototype experiment for checkingthe on-resistance of the semiconductor device having the layeredconfiguration formed of the Si layer/Ag layer/Ni layer or the Silayer/Ag layer versus the thickness of the Si layer. More specifically,FIG. 5A shows a graph illustrating the on-resistance of thesemiconductor device having the layered configuration formed of the Silayer/Ag layer versus the thickness of the Si layer in a case where theAg layer has thicknesses of 30 μm and 50 μm. In addition, FIG. 5A showsa graph illustrating the on-resistance of the semiconductor devicehaving the layered configuration formed of the Si layer/Ag layer/Nilayer versus the thickness of the Si layer in a case where the Ag layerhas the thickness of 30 μm and the Ni layer has a thickness of 30 μm andin a case where the Ag layer has the thickness of 50 μm and the Ni layerhas the thickness of 30 μm.

The on-resistance of each of the semiconductor devices decreases as thethickness of the Si layer decreases, as shown in FIG. 5A. It is,however, noted that a thinner Si layer allows reduction in on-resistancebut causes an increase in variation in film thickness on a semiconductorsubstrate wafer, local fracturing and cracking, and othermanufacturing-step-related problems. It is therefore difficult to stablyreduce the thickness of the Si layer to a value smaller than 20 μm.There are a tendency for the on-resistance to decrease as the thicknessof the Ag layer increases and a tendency for the on-resistance todecrease when the Ni layer is added.

FIG. 5B shows a graph of results of a prototype experiment for checkingthe on-resistance of the semiconductor device having the layeredconfiguration formed of the Si layer/Ag layer/Ni layer versus thethickness of the Ni layer. More specifically, FIG. 5B shows a graphillustrating the on-resistance versus the thickness of the Ni layer inthe case of the Si layer (20 μm)/Ag layer (30 μm) and the Si layer (20μm)/Ag layer (50 μm). FIG. 5B shows that the on-resistance of thesemiconductor device slightly decreases as the thickness of the Ni layerincreases, indicating that adding the Ni layer causes no increase in theon-resistance of the semiconductor device. In particular, in the casewhere the Si layer has the thickness of 20 μm, the Ag layer has thethickness of 30 μm, and the Ni layer has the thickness of 30 μm, theon-resistance decreases to about 2.3 mΩ.

FIGS. 5A and 5B also show that semiconductor device 1 having the layeredconfiguration formed of the Si layer/Ag layer/Ni layer with the Ag layerbeing thicker than the Si layer and the Ni layer being thicker than theSi layer allows both the reduction in the amount of warpage and lowon-resistance.

[4. Microscopic Configuration of Semiconductor Device]

In semiconductor device 1 according to the present embodiment, it isdesirable that the Ni layer is thicker than the Si layer and thethickness of the Si layer is greater than 20 μm, so that the Ni layerneeds to be a layer having a thickness of several tens of micrometers.From this point of view, the Ni layer is formed, for example, by wetplating. Wet plating is broadly classified into electroplating andchemical plating, and the electroplating is characterized by a smalldegree of restriction on the film thickness and low-temperatureformation of a film for a small amount of thermal influence on a device.The electroplating is therefore desirable as the method formanufacturing the Ni layer in semiconductor device 1. The method forforming the Ni layer can be dry method, such as evaporation, which has,however, a low film formation rate because the crystal grains have asize on the order of several tens of nanometers. The dry method istherefore impractical as the method for forming a thick film having athickness of 10 μm or greater.

In the electroplating, a potential gradient moves a metal seed ionizedin a solution toward the cathode, and the metal seed chemically combineswith the atoms of the base material of the cathode to form a metalcoating. The crystal grains of the formed metal coating therefore tendto grow into large crystal grains.

FIG. 6 shows the crystal state of the Ni layer in semiconductor device 1according to the embodiment. (a) in FIG. 6 is an electron micrograph ofprincipal surface 30 b of the Ni layer in semiconductor device 1, and(b) in FIG. 6 is an electron micrograph of a cross section of the Nilayer in semiconductor device 1.

In (a) in FIG. 6, principal surface 30 b of the Ni layer has anirregular structure formed of grains that are a plurality of aggregatedcrystals, and the irregular structure has a cycle ranging, for example,from 10 to 20 μm. On the other hand, (b) in FIG. 6 shows across-sectional structure of the Ni layer having a plurality of crystalgrains grown in a direction approximately perpendicular to principalsurface 30 b and each having a size greater than or equal to 1 μm.

(a) and (b) in FIG. 6 show a feature of thick metal layer 30 (Ni layer)formed by electroplating and specifically show that the horizontal cycle(cycle in x-axis direction) of the irregularities of principal surface30 b of metal layer 30 (Ni layer) is greater than the horizontal grainsize (size in x-axis direction) of the crystals that form the metallayer 30 (Ni layer).

The feature described above indicates that electroplating is effectiveas the method for forming an Ni layer thicker than the Si layer.

FIG. 7 is an electron micrograph showing a cross section of the Nilayer/Ag layer in semiconductor device 1 according to the embodiment.(a), (b), and (c) in FIG. 7 show cross sections of the Ag layer and theNi layer formed by electroplating in a case where plating current hasmagnitudes of 2.1 A, 4.5 A, and 8.0 A, respectively.

It has been known as a feature of a metal film that the crystallinity ofthe metal film is related to the hardness of the metal film, and that ametal film formed of finer crystal grains is harder (has a greaterYoung's modulus). Further, in electroplating, the farther the filmformation proceeds, the larger the crystal grains become. In otherwords, the thicker the film, the larger the crystal grains.

In semiconductor device 1 according to the present embodiment, the Nilayer/Ag layer is so formed that the Ag layer is formed on principalsurface 40 b of the Si layer and the Ni layer is then formed onprincipal surface 31 b of the Ag layer. As a result, irrespective of themagnitude of the plating current, the crystal grain size in metal layer30 (Ni layer) in the vicinity of principal surface 30 a is less than thecrystal grain size in metal layer 30 (Ni layer) in the vicinity ofprincipal surface 30 b, and the crystal grain size in metal layer 30 (Nilayer) in the vicinity of principal surface 30 a is less than thecrystal grain size in metal layer 31 (Ag layer) in the vicinity ofprincipal surface 31 b, as shown in FIG. 7.

As a result, since not only is the Ni layer having a relatively smallcoefficient of thermal expansion in contact with the Ag layer having arelatively large coefficient of thermal expansion but the Ni layerhaving a relatively small crystal grain size is in contact with the Aglayer having a relatively large crystal grain size, the Ag layer isunlikely to stretch when the temperature increases, whereby the effectof suppressing the warpage of semiconductor device 1 is enhanced.

FIG. 8 shows a graph illustrating comparison between actually measuredvalues and estimated values of the amount of warpage of thesemiconductor device having the layered configuration formed of the Silayer/Ag layer/Ni layer versus the thickness of the Ni layer. FIG. 8shows actually measured values and estimated values of the amount ofwarpage in the case where the Si layer has the thickness of 20 μm andthe Ag layer has the thickness of 50 μm in the layered configurationformed of the Si layer/Ag layer/Ni layer, with the actually measuredvalues provided in a prototype experiment and the predicted valuesprovided by a computer simulator when the thickness of the Ni layerincreases. FIG. 8 shows that the actually measured values of the amountof warpage deviate from the predicted values thereof in the region wherethe thickness of the Ni layer is 20 μm or greater, so that the warpagesuppression effect provided by an increase in the thickness of the Nilayer is degraded at the actually measured values in the region.

Increasing the thickness of the Ni layer enhances the effect ofsuppressing the warpage of semiconductor device 1 but prolongs theperiod required to form the Ni-plated layer, resulting in an increase inmanufacturing cost. Further, increasing the thickness of the Ni layerincreases the cutting load in the dicing step of singulation ofsemiconductor device 1, resulting in possible problems, such as anincrease in the manufacturing cost due to a decrease in cutting speedand breakage of the dicing blade.

It is therefore desirable from the viewpoint of an effective warpagesuppression effect and the manufacturing steps in relation to anincrease in the thickness of the Ni layer that the thickness of the Nilayer is smaller than or equal to 30 μm. That is, it is desirable thatthe metal layer 30 (Ni layer) is thicker than semiconductor layer 40 (Silayer) and the thickness of the metal layer 30 (Ni layer) is smallerthan or equal to 30 μm. As a result, an effective warpage suppressioneffect is provided, and shortening of the manufacturing steps and costreduction can be achieved.

FIG. 9 is a schematic cross-sectional view of semiconductor devices eachincluding an Ni layer formed of a plurality of layers containingdifferent crystal grain sizes.

(a) in FIG. 9 shows that the crystal grains in first layer 70A includingprincipal surface 30 a are smaller than the crystal grains in secondlayer 70B including principal surface 30 b. In the configurationdescribed above, first layer 70A is harder than second layer 70B.Therefore, as the tendency of the warpage in the Ni layer, soft secondlayer 70B is likely to stretch by a greater amount than hard first layer70A, and principal surface 30 b of the Ni layer has convex warpage, thedirection of which is the same as the direction of the warpage betweenthe Si layer and the Ag layer. That is, the layered Ni layer reduces theamount of warpage of semiconductor device 1, but the effect ofsuppressing the warpage of the semiconductor device is degraded due tothe crystal grain size distribution described above in the Ni layer.

The distribution of the crystal grain size in the Ni layer shown in (a)in FIG. 9 is a result of the formation of the Ni layer with the filmformation conditions in the electroplating unchanged. The configurationof the Ni layer shown in (a) in FIG. 9 can therefore suppress thewarpage of the semiconductor device while allowing the Ni layer to beformed by simplified electric plating, for example, the plating currentcondition is fixed.

(b) in FIG. 9 shows a state in which the crystal grain size in firstlayer 70C including principal surface 30 a is approximately equal to thecrystal grain size in second layer 70D including principal surface 30 b.Further, the crystal grain size in metal layer 30 (Ni layer) is lessthan the crystal grain size in a portion of metal layer 31 (Ag layer)that is the portion facing principal surface 31 b. The configurationdescribed above, in which the Ni layer has uniform hardness and the Nilayer is harder than the Ag layer, can suppress the warpage ofsemiconductor device 1. Further, since the Ni layer, which can be theouter surface of semiconductor device 1, is harder than the Ag layer,breakage of the dicing blade in the dicing-based cutting step can beavoided, whereby the manufacturing steps are simplified.

(c) in FIG. 9 shows a state in which the crystal grain in first layer70E including principal surface 30 a is greater than the crystal grainin second layer 70F including principal surface 30 b. In theconfiguration described above, first layer 70E is softer than secondlayer 70F. Therefore, as the tendency of the warpage in the Ni layer,soft first layer 70E is likely to stretch by a greater amount than hardsecond layer 70F, and principal surface 30 a of the Ni layer has convexwarpage, the direction of which is opposite the direction of the warpagebetween the Si layer and the Ag layer. That is, the crystal grain sizedistribution described above in the Ni layer enhances the effect ofsuppressing the warpage of semiconductor device 1. Further, since the Nilayer, which can be the outer surface of semiconductor device 1, has ahard portion facing the principal surface 30 b, the dicing-based cuttingstep is readily carried out, whereby the manufacturing steps aresimplified.

To achieve the crystal state in the first layer and the crystal state inthe second layer, as shown in (b) and (c) in FIG. 9, the plating currentconditions for the first and second layers may be set to differ fromeach other or may otherwise be individually controlled.

The relationship between the crystal orientation in metal layer 30 andthe hardness thereof will next be described.

It is generally known that the physical properties of a crystal having aregular atom arrangement change in accordance with the crystaldirection. The same holds true for Ni, and the physical properties of Nichange in accordance with the crystal orientation. A Young's modulus ofan Ni single crystal having a <110> crystal growth direction is2.04×10¹² (dyn/cm²), and a Young's modulus of an Ni single crystalhaving a <100> crystal growth direction is 1.21×10¹² (dyn/cm²). That is,the Ni layer having a <110> preferred orientation has a greater Young'smodulus and is harder than the Ni layer having a <100> preferredorientation.

In a case where an Ni layer is formed by using electroplating, it isknown that the crystal orientation of the Ni layer changes in accordancewith the solution used in the electroplating. For example, an Ni layerformed by using a sulfuric acid bath or a Watts bath has a crystalgrowth direction that coincides with the <110> preferred orientationover a relatively high plating current density range, and an Ni layerformed by using a sulfamic acid bath has a crystal growth direction thatcoincides with the <100> preferred orientation.

FIG. 10 shows the temperature dependence of the amount of warpage of asemiconductor device including an Ni layer formed of two layers formedby using different plating methods. (a) in FIG. 10 is a cross-sectionalview of a layered configuration in which a first layer (having thicknessof 15 μm) formed by using a sulfamic acid bath and a second layer(having thickness of 15 μm) formed by using a sulfuric acid bath areformed in the presented order from the side facing principal surface 31b of the Ag layer (having thickness of 50 μm). (b) in FIG. 10 shows thetemperature dependence of the amount of warpage in a configuration inwhich the Ag layer (having thickness of 50 μm) and the first layer(having thickness of 30 μm) are layered on each other, the temperaturedependence of the amount of warpage in a configuration in which the Aglayer (having thickness of 50 μm) and the second layer (having thicknessof 15 μm) are layered on each other, and the temperature dependence ofthe amount of warpage in a configuration in which the Ag layer (havingthickness of 50 μm), the first layer (having thickness of 15 μm), andthe second layer (having thickness of 15 μm) are layered on each other.

According to the findings described above, since the first layer(sulfamic acid Ni) has a crystal growth direction (direction towardnegative side of axis z) that coincides with the <100> preferredorientation and the second layer (sulfuric acid Ni) has a crystal growthdirection (direction toward negative side of axis z) that coincides withthe <110> preferred orientation, a Young's modulus of the second layeris greater than a Young's modulus of the first layer. The amount ofwarpage of the second layer therefore tends to be smaller than theamount of warpage of the first layer, and the upper limit of the amountof warpage lowers in a high-temperature region. The lower limit of theamount of warpage in a low-temperature region, however, strongly tendsto lower and reaches a negative region, so that the range between theupper and lower limits of the amount of warpage of the first layer isnarrower than that of the second layer. Layering the first and secondlayers having different characteristics of the amount of warpage on eachother allows the upper limit of the amount of warpage in thehigh-temperature region to be smaller than that of the first layer andthe lower limit (value in negative region) of the amount of warpage inthe low-temperature region to be greater than that of the second layer.

The term “preferred crystal orientation” is also called, for example, a<110> preferred orientation (or {110}-plane preferred orientation),indicates that the ratio of a crystal having a predetermined crystalorientation (or crystal plane) to the entire crystal per unit volume orarea is maximized, and can be determined by using X-ray diffraction orbackscattered electron diffraction.

According to the configuration described above, metal layer 30 (Nilayer) includes the first layer including principal surface 30 a and thesecond layer including principal surface 30 b, and the metal crystalincluded in the first layer and the metal crystal included in the secondlayer may have different preferred orientation planes in the horizontaldirection of principal surface 30 b.

The amount (the absolute value and range of the amount) of warpage andother parameters of semiconductor device 1 are thus readily controlled.

In the horizontal direction of principal surface 30 b, the metal crystalincluded in one of the first and second layers may have the {100}-planepreferred orientation, and the metal crystal included in the other oneof the first and second layers may have the {110}-plane preferredorientation.

The amount (the absolute value and range of the amount) of warpage andother parameters of semiconductor device 1 are thus readily controlled.

The metal crystal included in the metal layer 30 (Ni layer) may have the{100}-plane preferred orientation in principal surface 30 b irrespectiveof whether metal layer 30 (Ni layer) is formed of a single layer or aplurality of layers having different crystal orientations.

In this case, since a Young's modulus in the {100} plane is less than aYoung's modulus in the 11101 plane, laser marking in a marking step isreadily performed.

The metal crystal included in the metal layer 30 (Ni layer) may have the{110}-plane preferred orientation in principal surface 30 b irrespectiveof whether metal layer 30 (Ni layer) is formed of a single layer or aplurality of layers having different crystal orientations.

In this case, since a Young's modulus in the 11101 plane is greater thana Young's modulus in the {100} plane, the suppression of the warpage ofsemiconductor device 1 can be enhanced.

[5. Visibility of Mark on Semiconductor Device]

Semiconductor device 1 according to the present embodiment furtherincludes a mark formed on principal surface 30 b of metal layer 30 (Nilayer). The mark described above is, for example, a mark includingidentification information, such as the product name and the date ofmanufacture. In semiconductor device 1, the mark is formed on principalsurface 30 b, for example, by laser radiation so that extrinsic visualrecognition of the mark is readily allowed even after facedown mounting.A YAG laser is frequently used as the laser used to perform the laserradiation described above. A YAG laser is capable of minute marking on ametal material representatively including a resin material.

The visibility of the mark formed on principal surface 30 b of the Nilayer is greatly affected by the state of the surface of the Ni layer.Radiating laser onto principal surface 30 b of the Ni layer reconfiguresthe grain boundary in a region of principal surface 30 b that is theregion to which the laser is radiated and therefore changes the state ofprincipal surface 30 b. The relationship of the width (a pattern widthof the mark) and depth (a pattern depth of the mark) of a line of themark where the state of the surface has changed with the state of thesurface in a region of principal surface 30 b that is the region otherthan the line of the mark determines the visibility of the mark.Examples of deterioration of the visibility of the mark may include acase where it is difficult to identify the mark due, for example, toloss and blur of a marked letter or line.

FIG. 11 is a schematic cross-sectional view of the semiconductor deviceand shows the relationship of the cycle of irregularities of principalsurface 30 b of the Ni layer and the width of the mark pattern with thevisibility of the mark. In a case where the width of the pattern (inx-axis direction) in the line of the mark where the state of the surfacehas changed due to the radiation of the laser is less than the cycle ofthe irregularities of principal surface 30 b of the Ni layer, as shownin FIG. 11, the mark is visually unrecognizable and is thereforedetermined as defective. In contrast, in a case where the width of thepattern in the line of the mark described above is greater than thecycle of the irregularities of principal surface 30 b of the Ni layer,the mark is visually recognizable and is therefore determined assatisfactory.

FIG. 12 shows the relationship of the mean roughness depth of principalsurface 30 b of the Ni layer and the depth of the mark with thevisibility of the mark. In a case where the depth of the mark (in z-axisdirection) in the line of the mark where the state of the surface haschanged due to the radiation of the laser is less than a mean roughnessdepth Rz of principal surface 30 b of the Ni layer, as shown in FIG. 12,the mark is visually unrecognizable and is therefore determined asdefective. In contrast, in a case where the depth of the mark in theline of the mark described above is greater than the mean roughnessdepth Rz of principal surface 30 b of the Ni layer, the mark is visuallyrecognizable and is therefore determined as satisfactory.

[6. Configuration of End Portion of Semiconductor Device]

FIG. 13A is a cross-sectional view of semiconductor device 1A accordingto an embodiment. Semiconductor device 1A includes semiconductor layer40 (Si layer), metal layers 30 (Ni layer) and 31 (Ag layer), transistors10 and 20, and protrusions 36A, 36B, 37A and 37B, as shown in FIG. 13A.Semiconductor device 1A differs from semiconductor device 1 according tothe embodiment in that semiconductor device 1A includes protrusions 36A,36B, 37A and 37B, and that the outer periphery of the Si layer is closerto the center of semiconductor device 1A than the outer periphery of theNi layer and the Ag layer in the plan view of the Si layer.Semiconductor device 1A will be described below primarily on thedifferent points with no description of the same points as those ofsemiconductor device 1.

In the plan view of semiconductor layer 40 (Si layer), the outerperiphery of semiconductor layer 40 (Si layer) is closer to the centerof semiconductor device 1A than the outer peripheries of metal layers 30(Ni layer) and 31 (Ag layer). This results from the fact that the stepof singulation of semiconductor device 1A is formed of two cutting steps(step of cutting Si layer and step of cutting Ni and Ag layers). Insemiconductor device 1A, the configuration in which the outer peripheryof the Si layer is closer to the center than the outer peripheries ofthe Ni and Ag layers is not essential.

In the plan view of metal layer 30 (Ni layer) from the side facingprincipal surface 30 b, protrusions 36A and 36B are each a firstprotrusion that is located in the outer periphery of the Ni layer andprotrudes from principal surface 30 b in the direction from principalsurface 30 a toward principal surface 30 b (direction toward negativeside of z-axis direction). Protrusions 36A and 36B contain at least oneof the first metal material that metal layer 31 contains and the secondmetal material that metal layer 30 contains. In semiconductor device 1A,protrusions 36A and 36B contain at least one of Ag and Ni.

The mechanical strength and hardness of the Ni layer along the outerperiphery thereof can therefore be increased, whereby the suppression ofthe warpage of semiconductor device 1A can be enhanced.

Protrusions 36A and 36B may be formed on two sides facing each other outof the sides that form the outer periphery of metal layer 30 (Ni layer)or all sides that form the outer periphery of metal layer 30 (Ni layer)in the plan view described above.

The suppression of the warpage of semiconductor device 1A in thedirection perpendicular to the direction in which protrusions 36A and36B are formed can therefore be enhanced.

The protruding height of protrusions 36A and 36B is, for example, atleast one-third the thickness of metal layer 30 (Ni layer).

The mechanical strength and hardness of the Ni layer along the outerperiphery thereof can therefore be increased.

The protruding width of protrusions 36A and 36B is, for example, atleast 4 μm.

Therefore, the mechanical strength and hardness of the Ni layer alongthe outer periphery thereof can be increased, and a breakaway ofprotrusions 36A and 36B from semiconductor device 1A in a cleaning stepcan be avoided, whereby a short circuit and other defects that occur insemiconductor device 1A due to an electrically conductive object havingbroken away can be avoided.

In protrusions 36A and 36B, the content of the second metal material isgreater than the content of the first metal material. In semiconductordevice 1A, the Ni content is greater than the Ag content in protrusions36A and 36B.

Protrusions 36A and 36B are therefore relatively hard protrusionsbecause they contain a large amount of Ni, a Young's modulus of which isgreater than that of Ag. Therefore, the mechanical strength and hardnessof metal layer 30 along the outer periphery thereof can be increased,and a breakaway of protrusions 36A and 36B from semiconductor device 1Ain a cleaning step can be avoided, whereby a short circuit and otherdefects that occur in semiconductor device 1A due to an electricallyconductive object having broken away can be avoided.

In semiconductor device 1A, protrusions 36A and 36B are not an essentialconfiguration.

In the plan view of semiconductor layer 40 (Si layer), the outerperiphery of semiconductor layer 40 (Si layer) is formed inside theouter periphery of metal layer 31 (Ag layer) with a gap between the twoouter peripheries.

In the plan view of metal layer 31 (Ag layer) viewed from the sidefacing principal surface 31 a, protrusions 37A and 37B are each a secondprotrusion that is located at the outer periphery of metal layer 31 (Aglayer) and protrudes from principal surface 31 a in the direction fromprincipal surface 31 b toward principal surface 31 a (direction towardpositive side of z-axis direction). Protrusions 37A and 37B contain atleast one of the first metal material that metal layer 31 (Ag layer)contains and the second metal material that metal layer 30 (Ni layer)contains. In semiconductor device 1A, protrusions 37A and 37B contain atleast one of Ag and Ni.

The mechanical strength and hardness of the Ag layer along the outerperiphery thereof can therefore be increased, whereby the suppression ofthe warpage of semiconductor device 1A can be enhanced.

Protrusions 37A and 37B may be formed on two sides facing each other outof the sides that form the outer periphery of metal layer 31 (Ag layer)or all sides that form the outer periphery of metal layer 31 (Ag layer)in the plan view described above.

The suppression of the warpage of semiconductor device 1A in thedirection perpendicular to the direction in which protrusions 37A and37B are formed can therefore be enhanced.

In semiconductor device 1A, protrusions 37A and 37B are not an essentialconfiguration.

In semiconductor device 1A, protrusions 36A, 36B, 37A and 37B are eachmade of the material included in metal layer 30 (Ni layer) or metallayer 31 (Ag layer) and extends from the outer periphery of metal layer30 (Ni layer) or metal layer 31 (Ag layer) in the step of singulation ofsemiconductor device 1A.

In the step of singulation of semiconductor device 1A, blade dicing is,for example, used. The blade dicing is the process of cutting the Silayer, the Ni layer, and the Ag layer by rotating at high speed acircular blade to which diamond grindstones are attached and which has awidth of about several tens of micrometers. In this process, thecircular blade first cuts the Si layer and travels toward the Ni layerand removes the material of each of the layers by the amountcorresponding approximately to the width of the circular blade (severaltens of micrometers). The Ni and Ag layers, which are ductile, aretherefore stretched in the direction in which the circular bladeperforms the cutting, so that protrusions (burrs) are formed as part ofthe Ni and Ag layers. The protrusions form new, approximately flatsurfaces in the direction perpendicular to the surfaces of the Ni and Aglayers and serve as portions that reinforce the Ni and Ag layers. Thesuppression of warpage of semiconductor device 1A can therefore beenhanced.

FIG. 13B is a cross-sectional view of semiconductor device 1B accordingto an embodiment. Semiconductor device 1B includes semiconductor layer40 (Si layer), metal layers 30 (Ni layer) and 31 (Ag layer), andprotrusions 36B, 37B, and 38, as shown in FIG. 13B. Semiconductor device1B differs from semiconductor device 1A in that semiconductor device 1Bincludes protrusion 38. Semiconductor device 1B will be described belowprimarily on the different point with no description of the same pointsas those of semiconductor device 1A.

Protrusion 38 is a third protrusion formed in the outer peripheralsurface of metal layer 31 (Ag layer) in the direction from the center ofmetal layer 31 (Ag layer) toward the outer periphery thereof (directiontoward positive side of x-axis direction) in the plan view of metallayer 31 (Ag layer). Protrusion 38 may instead be formed on the outerperipheral surface of the Ni layer in the direction from the center ofthe Ni layer toward the outer periphery thereof (direction towardpositive side of x-axis direction) in the plan view of the Ni layer.

The mechanical strength and hardness of the Ni layer along the outerperiphery thereof can therefore be increased, whereby the suppression ofthe warpage of semiconductor device 1B can be enhanced.

FIG. 13C is a cross-sectional view of semiconductor device 1C accordingto an embodiment. Semiconductor device 1C includes semiconductor layer40 (Si layer), metal layers 30 (Ni layer) and 31 (Ag layer), cover layer50, protrusion 37A (not shown), and protrusion 37B, as shown in FIG.13C. Semiconductor device 1C differs from semiconductor device 1A inthat semiconductor device 1C includes no protrusion 36A or 36B but coverlayer 50. Semiconductor device 1C will be described below primarily onthe different points with no description of the same points as those ofsemiconductor device 1A.

Cover layer 50 is a first cover layer that includes principal surface 50a (seventh principal surface) and principal surface 50 b (eighthprincipal surface), which face in opposite directions, is disposed withprincipal surface 50 a in direct contact with principal surface 30 b ofthe Ni layer or formed via a bonding material, and comprises a ceramicor plastic material.

Cover layer 50 has been already disposed before the step of singulationof semiconductor device 1C. Cover layer 50 can prevent creation ofprotrusions (burrs) on principal surface 30 b of the Ni layer in thesingulation step using blade dicing.

FIG. 13D is a cross-sectional view of semiconductor device 1D accordingto an embodiment. Semiconductor device 1D includes semiconductor layer40 (Si layer), metal layers 30 (Ni layer) and 31 (Ag layer), and coverlayers 50 and 51, as shown in FIG. 13D. Semiconductor device 1D differsfrom semiconductor device 1C in that semiconductor device 1D includes noprotrusion 37A or 37B but cover layer 51. Semiconductor device 1D willbe described below primarily on the different points with no descriptionof the same points as those of semiconductor device 1C.

Cover layer 51 is a second cover layer that is located on an outer edgeportion of metal layer 31 (Ag layer) in the plan view of metal layer 31(Ag layer), includes principal surface 51 a (ninth principal surface)and principal surface 51 b (tenth principal surface), which face inopposite directions, is disposed with principal surface 51 b in directcontact with principal surface 31 a of metal layer 31 (Ag layer) orformed via a bonding material, and comprises a ceramic or plasticmaterial.

Cover layer 51 has been already disposed before the step of singulationof semiconductor device 1D. Cover layer 51 can prevent creation ofprotrusions (burrs) on principal surface 31 a of the Ag layer in thesingulation step using blade dicing.

FIG. 13E is a cross-sectional view of semiconductor device 1E accordingto an embodiment. Semiconductor device 1E includes semiconductor layer40 (Si layer), metal layers 30 (Ni layer) and 31 (Ag layer), cover layer50, and groove 60, as shown in FIG. 13E. Semiconductor device 1E differsfrom semiconductor device 1D in that semiconductor device 1E includes nocover layer 51 but groove 60, which divides the Si layer. Semiconductordevice 1E will be described below primarily on the different points withno description of the same points as those of semiconductor device 1D.

Groove 60 is a groove that is formed in an outer edge portion ofsemiconductor layer 40 (Si layer) and along the outer periphery ofsemiconductor layer 40 (Si layer) and has a bottom surface that isprincipal surface 31 a.

Groove 60 has been already formed before the step of singulation ofsemiconductor device 1E. Groove 60 can prevent chippings from the Silayer that reach body regions 18 and 28 in the singulation step usingblade dicing.

FIG. 13F is a cross-sectional view of semiconductor device 1F accordingto an embodiment. Semiconductor device 1F includes semiconductor layer40 (Si layer), metal layers 30 (Ni layer) and 31 (Ag layer), andcombined object 39, as shown in FIG. 13F. Semiconductor device 1Fdiffers from semiconductor device 1 in that semiconductor device 1Fincludes combined object 39. Semiconductor device 1F will be describedbelow primarily on the different point with no description of the samepoints as those of semiconductor device 1.

Combined object 39 is a combined object that is the combination of thefirst metal material and the second metal material and is formed on theouter peripheral surface of metal layer 30 (Ni layer). In the presentembodiment, combined object 39 is the combination of Ag and Ni.

The mechanical strength and hardness of the Ni layer along the outerperiphery thereof can therefore be increased, whereby the suppression ofthe warpage of semiconductor device 1F can be enhanced. Combined object39 may instead be formed on the outer peripheral surface of the Aglayer. Also in this case, the mechanical strength and hardness of the Aglayer along the outer periphery thereof can be increased, whereby thesuppression of the warpage of semiconductor device 1F can be enhanced.

In the step of singulation of semiconductor device 1F, laser dicing is,for example, used as the mean for cutting the Ni and Ag layers. Thecombination of the first and second metal materials melted by the lasertherefore adheres to the outer peripheral surfaces of the Ni and Aglayers.

In the plan view of metal layers 30 (Ni layer) and 31 (Ag layer),combined object 39 may be formed on the outer peripheral surface of atleast one of metal layers 30 (Ni layer) and 31 (Ag layer) throughout theentire periphery of semiconductor device 1F.

The mechanical strength and hardness of the Ni layer or the Ag layeralong the outer periphery thereof can therefore be further increased.

In the direction from principal surface 31 a toward principal surface 30b (direction toward negative side of z-axis direction), the centerposition of combined object 39 is located, for example, between theposition corresponding to half the distance from principal surface 31 ato principal surface 30 b and the position of principal surface 30 b, asshown in FIG. 13F.

Thus formed combined object 39 can avoid adhesion of free objects thatare objects that form the Ni and Ag layers scattered in the singulationstep using laser dicing (hereinafter referred to as debris in somecases) to the surface of semiconductor device 1F (surface facingpositive side of z-axis direction). For example, when the combined layerthat is the combination of the Ni layer and the Ag layer is cut, theblade dicing is performed from the side facing principal surface 31 a bya thickness equal to at least half the distance from principal surface31 a to principal surface 30 b, and then the remaining thin combinedlayer undergoes laser dicing at low-intensity laser output from the sidefacing principal surface 31 a or 30 b. Creation of debris can thus besuppressed.

It is acceptable that protrusions 36A, 36B, 37A, 37B, and 38 andcombined object 39 are continuously formed over the distancecorresponding to at least one-third of each outer peripheral side ofsemiconductor device 1A. The configuration described above in which aprotrusion or a combined object is continuously formed along each outerperipheral side of semiconductor device 1A enhances suppression of thewarpage of semiconductor device 1A. Further, a protrusion or a combinedobject is continuously formed over the distance corresponding to atleast one half or two-thirds of each outer peripheral side ofsemiconductor device 1A in some cases depending on the conditions of thesingulation step. In this case, the suppression of the warpage ofsemiconductor device 1A is further enhanced.

FIG. 14 is a cross-sectional view of semiconductor device 1G accordingto an embodiment. Semiconductor device 1G includes semiconductor layer40 (Si layer), metal layers 30 (Ni layer) and 31 (Ag layer), andtransistors 10 and 20, as shown in FIG. 14. In the plan view ofsemiconductor layer 40 (Si layer), semiconductor device 1G differs fromsemiconductor device 1 in that the outer periphery of semiconductorlayer 40 (Si layer) is closer to the center of semiconductor device 1Gthan the outer peripheries of metal layers 30 (Ni layer) and 31 (Aglayer). Semiconductor device 1G will be described below primarily on thedifferent point with no description of the same points as those ofsemiconductor device 1.

In the plan view of semiconductor layer 40 (Si layer), the outerperiphery of semiconductor layer 40 (Si layer) is formed inside theouter periphery of metal layer 31 (Ag layer) with a gap between the twoouter peripheries. Further, the outer periphery of semiconductor layer40 (Si layer) may be formed inside the outer periphery of metal layer 31(Ag layer) with a gap between the two outer peripheries throughout theentire periphery of metal layer 31 (Ag layer).

The configuration described above has been already formed before the Niand Ag layers are cut in the singulation step. The configurationdescribed above can prevent chippings from the Si layer and adhesion ofdebris to the side surface of the Si layer when the Ni and Ag layers arediced. Further, when the Ni and Ag layers undergo blade dicing, the Silayer does not need to be cut at the same time, whereby the blade dicingcutting load can be lowered, and the dicing blade used in the dicing isreadily selected. The reason for this is that the type of dicing bladesuitable for cutting of the Si layer, which is made of a ceramicmaterial, differs from the type of dicing blade suitable for cutting ofthe Ni and Ag layers, which are each made of a metal material.

The distance between the outer periphery of semiconductor layer 40 (Silayer) and the outer periphery of metal layer 31 (Ag layer) in thedirection in the planes to which semiconductor layer 40 (Si layer) andmetal layer 31 (Ag layer) belong (length of gap between outerperipheries) is, for example, at least 15 μm.

FIG. 15 describes the setback distance of the Si layer in semiconductordevice 1G according to the embodiment. FIG. 15 is a cross-sectional viewof the boundary region between two adjacent semiconductor devices 1G ina manufacturing step.

In the step of singulation of semiconductor device 1G, the Si layer ischipped in some cases when the Si layer undergoes the blade dicing. Toavoid the chipping, it is effective to perform the singulation ofsemiconductor device 1G by using not only the blade dicing but plasmadicing. Plasma dicing is a dry etching method for chemically removingthe Si layer based on a plasma reaction and can cut the Si layer with nochipping at a cut surface of the Si layer.

In the step of singulation of semiconductor device 1G, the plasma dicingremoves the Si layer in a region that undergoes the blade dicing orlaser dicing later and corresponds to a cutting width (plasma processingwidth in FIG. 15) that is the cutting width in the following bladedicing or laser dicing (laser processing width or blade processing widthin FIG. 15) plus a margin width. The Ag and Ni layers are then cut inthe blade or laser dicing. The singulation of semiconductor device 1Gcan thus be performed with no chipping at the outer periphery of the Silayer.

To avoid damage to the Si layer in the blade or laser dicing when the Agand Ni layers are cut, it is necessary to set the gap between the Silayers of adjacent semiconductor devices 1G at a value greater than thelaser or blade processing width. The outer periphery of each of the Silayers is thus formed inside the outer periphery of the Ag layer with agap therefrom.

In a case where the laser or blade processing width is set at a valueranging, for example, from 30 to 35 μm and the processing width in theplasma dicing is set, for example, at 65 μm, the distance between theouter periphery of the Si layer and the outer periphery of the Ag layer(setback distance LB in FIG. 15) ranges, for example, from 15 to 17 μm,as shown in FIG. 15.

Therefore, as the step of singulation of semiconductor device 1G,singulation with no chipping at the outer periphery of the Si layer canbe readily achieved by processing the Si layer in the plasma dicing andprocessing the Ag and Ni layers in the blade or laser dicing.

FIG. 16 is an electron micrograph of the side surface of the Si layer insemiconductor device 1G according to the embodiment. Mean roughnessdepth Rz of the irregularities of a first portion of the outerperipheral surface of semiconductor layer 40 (Si layer) is approximatelyequal to mean roughness depth Rz of the irregularities of a secondportion of the outer peripheral surface of semiconductor layer 40 (Silayer), as shown in FIG. 16, the first portion being in contact withprincipal surface 31 a and on a side facing principal surface 40 b, thesecond portion being on a side facing principal surface 40 a. The firstportion of the outer peripheral surface of semiconductor layer 40 (Silayer) is not formed in the region outside the outermost periphery ofthe second portion of the outer peripheral surface of semiconductorlayer 40 (Si layer) in the plan view of semiconductor layer 40 (Silayer). That is, no residue of semiconductor layer 40 is formed onprincipal surface 31 a of the Ag layer.

Therefore, when the Ag and Ni layers undergo the laser dicing, adhesionof the metals that form the Ag and Ni layers to the side surface of theSi layer can be avoided.

The outer peripheral surface of semiconductor layer 40 (Si layer) mayhave an irregular shape containing acute vertices, as shown in FIG. 16.

In this case, the irregular shape containing acute vertices facilitatesheat dissipation via the outer peripheral surface of the Si layer,whereby the heat dissipation capability of semiconductor device 1G isimproved.

FIG. 17 describes the relationship between the shape of the side surfaceof the Si layer and the method for manufacturing the Si layer insemiconductor device 1G according to the embodiment. (a) in FIG. 17shows the irregular shape containing acute vertices based on the shapeof a mask for the plasma dicing in the plan view of the Si layer. (b) inFIG. 17 shows the state of the processing in the plasma dicing in across-sectional view of the Si layer. In the plasma dicing step in (b)in FIG. 17, the Si layer is processed in plasma cutting performed inmultiple stages, so that irregularities containing acute vertices areformed on the side surface of the Si layer both in the y-axis direction(or x-axis direction) and the z-axis direction. Mean roughness depth Rzof the irregularities of a first portion of the outer peripheral surfaceof the Si layer is approximately equal to mean roughness depth Rz of theirregularities of a second portion of the outer peripheral surface ofthe Si layer, as shown in (b) in FIG. 17, the first portion being incontact with the Ag layer, the second portion being on the side oppositeto the Ag layer. Further, the first portion of the outer peripheralsurface of the Si layer is not formed in the region outside theoutermost periphery of the second portion of the outer peripheralsurface of the Si layer in the plan view of the Si layer.

FIG. 18 is an electron micrograph of the side surfaces of the Si layerand the Ag layer in semiconductor device 1H according to an embodiment.Semiconductor device 1H includes semiconductor layer 40 (Si layer),metal layers 30 (Ni layer) and 31 (Ag layer), transistors 10 and 20, andamorphous semiconductor 44, as shown in FIG. 18. Semiconductor device 1Hdiffers from semiconductor device 1G in that semiconductor device 1Hincludes amorphous semiconductor 44. Semiconductor device 1H will bedescribed below primarily on the different point with no description ofthe same points as those of semiconductor device 1G.

Amorphous semiconductor 44 is formed in the outer peripheral surface ofsemiconductor layer 40 (Si layer) to cover semiconductor layer 40 (Silayer).

Amorphous semiconductor 44 is made of Si included in the Si layer andmelts, solidifies, and adheres to the side surface of the Si layer whenthe Si layer undergoes laser dicing in the step of singulation ofsemiconductor device 111. Cracks in the Si layer and separation of partof the Si layer are therefore avoided when the Ag and Ni layers undergothe following blade dicing.

FIG. 19 is a cross-sectional view of semiconductor device 1J accordingto an embodiment. Semiconductor device 1J includes semiconductor layer40 (Si layer), metal layers 30 (Ni layer) and 31 (Ag layer), transistors10 and 20, and grooves 43A and 43B, as shown in FIG. 19. Semiconductordevice 1J differs from semiconductor device 1G in that semiconductordevice 1J includes grooves 43A and 43B. Semiconductor device 1J will bedescribed below primarily on the different point with no description ofthe same points as those of semiconductor device 1G.

Transistor 10 includes a plurality of grooves 41 (plurality of firstgrooves, main body trenches), which are filled with a solid componentincluded in gate conductor 15 and gate insulating film 16, in principalsurface 40 a of the Si layer.

Transistor 20 includes a plurality of grooves 42 (plurality of secondgrooves, main body trenches), which are filled with a solid componentincluded in gate conductor 25 and gate insulating film 26, in principalsurface 40 a of the Si layer.

The plurality of grooves 43A and the plurality of grooves 43B are aplurality of third grooves (dummy trenches) formed in an outer edgeportion of principal surface 40 a of semiconductor layer 40 (Si layer)along the outer peripheral side of semiconductor layer 40 (Si layer) andfilled with a solid component containing silicon. The plurality ofgrooves 43A are disposed in an outer edge portion facing transistor 10out of the above-mentioned outer edge portion of principal surface 40 a.The plurality of grooves 43B are disposed in an outer edge portionfacing transistor 20 out of the above-mentioned outer edge portion ofprincipal surface 40 a.

The solid component with which the plurality of grooves 43A and theplurality of grooves 43B are filled may be made of the same material asthe solid component with which the plurality of grooves 41 and theplurality of grooves 42 are filled. In this case, the plurality ofgrooves 43A and the plurality of grooves 43B can be formed in the samestep as the step of forming the plurality of grooves 41 and theplurality of grooves 42. The manufacturing steps can thus be simplified.

Therefore, in the step of singulation of semiconductor device 1J, asituation in which cracks in the Si layer and partial separation of theSi layer that occur in the blade dicing reach body regions 18 and 28 canbe avoided.

Gap LgA between the plurality of grooves 41 and the plurality of grooves43A and gap LgB between the plurality of grooves 42 and the plurality ofgrooves 43B may be greater than gap Pg1 between adjacent ones of theplurality of grooves 41 and may be greater than gap Pg2 between adjacentones of the plurality of grooves 42.

Therefore, since gaps LgA and LgB are greater than gaps Pg1 and Pg2between the grooves in the main body, the occupancy factor of a maskpattern for manufacturing semiconductor device 1J can be reduced to fallwithin a stably manufacturable range.

Semiconductor device 1J further includes passivation layer 35(protection layer) formed to overlap with part of source electrode 11 or21. In the plan view of semiconductor layer 40 (Si layer), the outerperiphery of passivation layer 35 may be formed inside the outerperiphery of semiconductor layer 40 (Si layer) with a gap between theouter peripheries, and the plurality of grooves 43A and the plurality ofgrooves 43B may be formed in the segment between the outer periphery ofsemiconductor layer 40 (Si layer) and the outer periphery of passivationlayer 35 in the plan view described above.

Resistance to cracks in the Si layer and partial separation of the Silayer that occur in the blade dicing can therefore be improved also inthe outer edge portion of principal surface 40 a of the Si layer, whereno passivation layer 35 is formed so that resistance to cracks in the Silayer and partial separation of the Si layer is poor.

The interval between the plurality of grooves 43A and the plurality ofgrooves 43B may be equal to the interval between the plurality ofgrooves 41 and the plurality of grooves 42.

The plurality of grooves 43A and the plurality of grooves 43B cantherefore be formed at the same time in the step of forming theplurality of grooves 41 and the plurality of grooves 42, whereby thedesign and manufacture of semiconductor device 1J can be simplified.

OTHER EMBODIMENTS

Although a semiconductor device according to one or more aspects of thepresent disclosure has been described based on an embodiment, thepresent disclosure is not limited to this embodiment. Those skilled inthe art will readily appreciate that embodiments arrived at by makingvarious modifications to the above embodiment or embodiments arrived atby selectively combining elements disclosed in the above embodimentwithout materially departing from the scope of the present disclosuremay be included within one or more aspects of the present disclosure.

In the present embodiment, semiconductor device 1, in which two verticalMOS transistors are formed in a semiconductor substrate comprisingsilicon, has been presented by way of example. The semiconductor deviceaccording to the present invention includes a semiconductor devicehaving the following configuration.

FIG. 20 is a schematic cross-sectional view of the portion correspondingto one vertical III-group-nitride semiconductor transistor included insemiconductor device 100 according to an embodiment. Semiconductordevice 100 is a chip-size-package-type III-group-nitride semiconductortransistor that allows facedown mounting. The vertical III-group-nitridesemiconductor transistor included in semiconductor device 100 includessubstrate 132, which comprises an n-type III-group-nitridesemiconductor, n-type III-group-nitride semiconductor layer 133 (and143), p-type III-group-nitride semiconductor layer 134 (and 144), andmetal layers 130 and 131, as shown in FIG. 20. Semiconductor device 100is provided with a recess passing through part of III-group-nitridesemiconductor layer 134 and having a bottom that reachesIII-group-nitride semiconductor layer 133. Semiconductor device 100 isfurther provided with a recess passing through part of III-group-nitridesemiconductor layer 144 and having a bottom that reachesIII-group-nitride semiconductor layer 143. Further, III-group-nitridesemiconductor layer 137 and III-group-nitride semiconductor layer 135,which has a bandgap wider than that of III-group-nitride semiconductorlayer 137, are sequentially formed to cover the bottom and the side ofeach of the recesses and part of the surface of III-group-nitridesemiconductor layer 134. III-group-nitride semiconductor layer 147 andIII-group-nitride semiconductor layer 145, which has a bandgap widerthan that of III-group-nitride semiconductor layer 147, are sequentiallyformed to cover the bottom and the side of each of the recesses and partof the surface of III-group-nitride semiconductor layer 144. Further,gate conductor 119 is formed on the surface of III-group-nitridesemiconductor layer 134, and source electrode 111 is formed on the upperlayer of III-group-nitride semiconductor layer 135. Further, gateconductor 129 is formed on the surface of III-group-nitridesemiconductor layer 144, and source electrode 121 is formed on the upperlayer of III-group-nitride semiconductor layer 145. Two-dimensionalelectron gas 136 is created in the vicinity of the boundary betweenIII-group-nitride semiconductor layer 137 and III-group-nitridesemiconductor layer 135. Two-dimensional electron gas 146 is created inthe vicinity of the boundary between III-group-nitride semiconductorlayer 147 and III-group-nitride semiconductor layer 145.

Semiconductor layer 140, which is the laminate of III-group-nitridesemiconductor layer 134, III-group-nitride semiconductor layer 133, andsubstrate 132, includes a first principal surface and a second principalsurface that face in opposite directions. Semiconductor layer 150, whichis the laminate of III-group-nitride semiconductor layer 144,III-group-nitride semiconductor layer 143, and substrate 132, includes afirst principal surface and a second principal surface that face inopposite directions.

Metal layer 131 is a first metal layer that includes a third principalsurface and a fourth principal surface that face in opposite directions,is disposed with the third principal surface in contact with the secondprincipal surface, is thicker than semiconductor layers 140 and 150, andcomprises the first metal material.

Metal layer 130 is a second metal layer that includes a fifth principalsurface and a sixth principal surface that face in opposite directions,is disposed with the fifth principal surface in contact with the fourthprincipal surface, is thicker than semiconductor layers 140 and 150, andcomprises the second metal material having a Young's modulus greaterthan that of the first metal material.

A first vertical III-group-nitride semiconductor transistor formed ofsubstrate 132, III-group-nitride semiconductor layers 133, 134, 135, and137, metal layers 130 and 131, gate conductor 119, and source electrode111 is formed in a first region of semiconductor layer 140, and a secondvertical III-group-nitride semiconductor transistor formed of substrate132, III-group-nitride semiconductor layers 143, 144, 145, and 147,metal layers 130 and 131, gate conductor 129, and source electrode 121is formed in a second region adjacent to the first region in thedirection along the first principal surface.

Metal layer 131 functions as a common drain electrode to the first andsecond vertical III-group-nitride semiconductor transistors.

III-group-nitride semiconductor layers 133 and 143 may be a continuoussingle layer. III-group-nitride semiconductor layers 135 and 145 may bea continuous single layer. III-group-nitride semiconductor layers 137and 147 may be a continuous single layer. Two-dimensional electron gases136 and 146 may be continuous with each other. Moreover, a group 3element included in III-group-nitride semiconductor layers 133, 134,135, 137, 143, 144, 145, and 147 may be Al, Ga, or In, or a combinationof these.

According to the configuration described above, in which metal layer 131having a thickness for ensuring low on-resistance is in contact withmetal layer 130 having a Young's modulus greater than that of metallayer 131 and thicker than semiconductor layers 140 and 150, warpage ofsemiconductor device 100 that occurs due to the contact of semiconductorlayers 140 and 150 with metal layer 131 can be suppressed.Chip-size-package-type semiconductor device 100 that allows bothreduction in on-resistance and suppression of warpage can therefore beprovided.

FIG. 21 is a schematic cross-sectional view of the portion correspondingto one vertical SiC transistor included in semiconductor device 200according to an embodiment. Semiconductor device 200 is achip-size-package-type SiC (silicon carbide) power transistor thatallows facedown mounting. The SiC transistor included in semiconductordevice 200 includes SiC substrate 232 containing a high-concentrationn-type impurity, low-concentration n-type impurity layer 233, and metallayers 230 and 231, as shown in FIG. 21. Low-concentration n-typeimpurity layer 233 is provided with p-type impurity layers each having ahigh-concentration n-type impurity layer formed therein. Sourceelectrode 211 (and 221) is provided on the surfaces of thehigh-concentration n-type impurity layers in the p-type impurity layersand the surfaces of the p-type impurity layers to be in contacttherewith, and gate conductor 219 (and 229) is provided via insulatingfilm 216 in a position where gate conductor 219 (and 229) faces thep-type impurity layers between high-concentration n-type impurity layersin the p-type impurity layers and low-concentration n-type impuritylayer 233.

Semiconductor layer 240, which is the laminate of low-concentrationn-type impurity layer 233 and SiC substrate 232, includes a firstprincipal surface and a second principal surface that face in oppositedirections.

Metal layer 231 is a first metal layer that includes a third principalsurface and a fourth principal surface that face in opposite directions,is disposed with the third principal surface in contact with the secondprincipal surface, is thicker than semiconductor layer 240, andcomprises the first metal material.

Metal layer 230 is a second metal layer that includes a fifth principalsurface and a sixth principal surface that face in opposite directions,is disposed with the fifth principal surface in contact with the fourthprincipal surface, is thicker than semiconductor layer 240, andcomprises the second metal material having a Young's modulus greaterthan that of the first metal material.

A first vertical SiC transistor formed of SiC substrate 232,low-concentration n-type impurity layer 233, metal layers 230 and 231,gate conductor 219, and source electrode 211 is formed in a first regionof semiconductor layer 240, and a second vertical SiC transistor formedof SiC substrate 232, low-concentration n-type impurity layer 233, metallayers 230 and 231, gate conductor 229, and source electrode 221 isformed in a second region adjacent to the first region in the directionalong the first principal surface.

Metal layer 231 functions as a common drain electrode to the first andsecond vertical SiC transistors.

According to the configuration described above, in which metal layer 231having a thickness for ensuring low on-resistance is in contact withmetal layer 230 having a Young's modulus greater than that of metallayer 231 and thicker than semiconductor layer 240, warpage ofsemiconductor device 200 that occurs due to the contact of semiconductorlayer 240 with metal layer 231 can be suppressed. Chip-size-package-typesemiconductor device 200 that allows both reduction in on-resistance andsuppression of warpage can therefore be provided.

INDUSTRIAL APPLICABILITY

The semiconductor devices according to the invention of the presentapplication in the form of a CSP-type semiconductor device can each bewidely used as a variety of semiconductor devices formed ofbidirectional transistors.

REFERENCE MARKS IN THE DRAWINGS

-   -   1, 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H, 1J, 100, 200 semiconductor        device    -   2 control IC    -   3 battery    -   4 load    -   10 transistor (first vertical MOS transistor)    -   11, 11 a, 11A, 11 b, 11B, 11 c, 11 d, 21, 21 a, 21A, 21 b, 21B,        21 c, 21 d, 111, 121, 211, 221 source electrode    -   12, 13, 22, 23 portion    -   14, 24 source region    -   15, 25, 119, 129, 219, 229 gate conductor    -   16, 26 gate insulating film    -   18, 28 body region    -   19, 19A, 19B, 29, 29A, 29B gate electrode    -   20 transistor (second vertical MOS transistor)    -   30, 31, 130, 131, 230, 231 metal layer    -   30 a, 30 b, 31 a, 31 b, 40 a, 40 b, 50 a, 50 b, 51 a, 51 b        principal surface    -   32 semiconductor substrate    -   33 low-concentration impurity layer    -   34 interlayer insulating layer    -   35 passivation layer    -   36A, 36B, 37A, 37B, 38 protrusion    -   39 combined object    -   40, 140, 150, 240 semiconductor layer    -   41, 42, 43A, 43B, 60 groove    -   44 amorphous semiconductor    -   50, 51 cover layer    -   70A, 70C, 70E first layer    -   70B, 70D, 70F second layer    -   132 substrate    -   133, 134, 135, 137, 143, 144, 145, 147 III-group-nitride        semiconductor layer    -   136, 146 two-dimensional electron gas    -   216 insulating film    -   232 SiC substrate    -   233 low-concentration n-type impurity layer

1. A semiconductor device which is a chip-size-package-typesemiconductor device that is facedown mountable, the semiconductordevice comprising: a semiconductor layer that includes a first principalsurface and a second principal surface that face in opposite directions,and comprises silicon, gallium nitride, or silicon carbide; a firstmetal layer that includes a third principal surface and a fourthprincipal surface that face in opposite directions, is disposed with thethird principal surface in contact with the second principal surface, isthicker than the semiconductor layer, and comprises a first metalmaterial; a second metal layer that includes a fifth principal surfaceand a sixth principal surface that face in opposite directions, andcomprises a second metal material having a Young's modulus greater thana Young's modulus of the first metal material; a first vertical fieldeffect transistor disposed in a first region of the semiconductor layer;and a second vertical field effect transistor disposed in a secondregion adjacent to the first region in a direction along the firstprincipal surface in the semiconductor layer, wherein the first verticalfield effect transistor includes a first source electrode and a firstgate electrode on a side facing the first principal surface of thesemiconductor layer, the second vertical field effect transistorincludes a second source electrode and a second gate electrode on theside facing the first principal surface of the semiconductor layer, thefirst metal layer functions as a common drain electrode for the firstvertical field effect transistor and the second vertical field effecttransistor, and a bidirectional path from the first source electrode tothe second source electrode via the common drain electrode is a primarycurrent path.
 2. The semiconductor device according to claim 1, whereinthe second metal layer has a thickness of at least 2 μm.
 3. Thesemiconductor device according to claim 1, wherein a coefficient ofthermal expansion of the second metal material is less than acoefficient of thermal expansion of the first metal material.
 4. Thesemiconductor device according to claim 1, further comprising: a firstprotrusion that is located in an outer periphery of the second metallayer in a plan view of the second metal layer and protrudes from thesixth principal surface in a direction from the fifth principal surfacetoward the sixth principal surface, wherein the first protrusioncontains at least one of the first metal material and the second metalmaterial.
 5. The semiconductor device according to claim 4, wherein thefirst protrusion is located on each of two opposing outer peripheralsides of the second metal layer in the plan view.
 6. The semiconductordevice according to claim 4, wherein a protruding height of the firstprotrusion is at least ⅓ of a thickness of the second metal layer. 7.The semiconductor device according to claim 4, wherein a protrudingwidth of the first protrusion is at least 4 μm.
 8. The semiconductordevice according to claim 1, further comprising: a first cover layerthat includes a seventh principal surface and an eighth principalsurface that face in opposite directions, is disposed with the seventhprincipal surface in contact with the sixth principal surface, andcomprises a ceramic or plastic material.
 9. The semiconductor deviceaccording to claim 1, further comprising: a combined object that is acombination of the first metal material and the second metal materialand is located in an outer peripheral surface of at least one of thefirst metal layer and the second metal layer.
 10. The semiconductordevice according to claim 9, wherein the combined object is located inthe outer peripheral surface over an entire periphery of thesemiconductor device in a plan view of the first metal layer and thesecond metal layer.
 11. The semiconductor device according to claim 9,wherein in a direction from the third principal surface toward the sixthprincipal surface, a position of a center of the combined object isbetween a position corresponding to half a distance from the thirdprincipal surface to the sixth principal surface and a position of thesixth principal surface.
 12. The semiconductor device according to claim1, wherein an outer periphery of the semiconductor layer is locatedinside an outer periphery of the first metal layer with a gap betweenthe outer peripheries in a plan view of the semiconductor layer.
 13. Thesemiconductor device according to claim 12, wherein the outer peripheryof the semiconductor layer is located inside the outer periphery of thefirst metal layer over an entire periphery of the semiconductor devicewith the gap between the outer peripheries in the plan view.
 14. Thesemiconductor device according to claim 12, wherein the gap has a lengthof at least 15 μm.